Development of Lentil Aquafaba-Based Food Emulsions with Xanthan Gum or Pregelatinized Corn Starch as Stabilizers ^†

Abstract

Currently, there is an increasing trend towards the use of legumes aquafaba-based emulsions for food applications. In this study, emulsions containing 30 and 60% sunflower oil with lentil aquafaba (LA) were developed, and xanthan or pregelatinized corn starch were added as stabilizers. Preliminary studies of lentil technological properties enabled the optimization of aquafaba production, achieving a dry matter content of 5.5% and a protein concentration of 1.1%. Emulsions with 0.5 and 0.8% aquafaba lentil protein without and with the addition of xanthan gum (0.1 and 0.2%) or starch (1 and 2%) were studied. Increasing the xanthan and starch content resulted in an increase in the average droplet size for emulsions with 30% oil and a decrease in the values for emulsions with 60% oil. For emulsions with a lower oil content, there was a visual instability over time with the addition of starch, which led to emulsion degradation. Rheological studies made it possible to classify the samples as a non-Newtonian fluid with a pseudoplastic flow pattern. The stability of the emulsions was observed due to an increase in the viscosity of the continuous phase due to the inclusion of the stabilizer. The influence of the nature of the stabilizer on this process is confirmed by calculations using various rheological models. Food emulsions obtained using lentil aquafaba are a promising ingredient in the development of emulsion food formulations.

1. Introduction

Humanity’s growing need for protein and the search for alternative protein sources, coupled with the expansion of the range of food products that satisfy various ethical, environmental, and religious restrictions on the consumption of animal proteins, are the cur-rent challenges for food science. The utilization of plant protein in food technology is now considered a new trend [1]. This approach makes it possible to solve the problems described above, as well as satisfy the increased tendency to consume plant products among the world’s population as a result of increasing awareness of the positive impact of these products on health and the general principles of the concept of sustainable development. It is well known that plant protein has the advantages of sustainability, low cost, and high efficiency compared to animal protein [2].

In the last decade, food science has seen a steady increase in research on the use of aquafaba as a by-product obtained from plant material/legume grains [3,4]. Aquafaba is defined as a liquid obtained by hydrothermal treatment of legume grains using traditional methods or under pressure. The latter method is most often used in the technology of sterilized products, such as canned chickpeas, beans, and peas [5].

Legumes aquafaba has a neutral flavor, is gluten-free and cholesterol-free, and has the necessary physicochemical properties to reproduce the structure and texture of animal protein. The qualitative composition of aquafaba (including soluble proteins, saponins, starch and its degradation products, and dietary fibers) and its quantitative composition (mass fractions of proteins, complex carbohydrates, and other substances) make it a versatile food ingredient with a wide range of functional properties [6,7,8,9]. Due to the presence of surfactants, aquafaba is widely used in the production of food products with foam and emulsion structures. A crucial aspect of aquafaba usage lies not only in the formation of emulsions or foams but also in ensuring their physical stability, as well as the sensory and rheological characteristics of the final product [10].

Currently, chickpea-based aquafaba is the most popular and widely used in the production of ice cream, soufflés, and baking [3], and in the development of plant-based mayonnaises [8]. Alongside the usage of chickpea in food production, other legumes such as lentils are also commonly used in the food industry. The sustainable production, low-cost, and large-scale cultivation and processing of lentils highlight the potential of lentil-based aquafaba, which necessitates further research.

According to the above, the purpose of this study was to develop food emulsions based on lentil aquafaba. The use of lentils (Lens culinaris) as a legume has several advantages. This plant has a higher protein content than other legumes such as chickpeas or peas [11,12], which is important for food emulsion stabilization [13,14]. Additionally, for physical stabilization of the obtained emulsions, stabilizers of a different nature were added, xanthan and pregelatinized corn starch. The developed emulsions are supposed to be used in emulsion-light food technologies.

2. Materials and Methods

2.1. Materials

Red lentils (variety Canadian, region of origin Odessa region, Ukraine) and sunflower oil (refined, deodorized, frozen out, 0.05–0.06% w/w moisture content, Oleina^TM, DP Suntrade, Kyiv, Ukraine) were obtained from local stores. Commercial xanthan gum Ziboxan F200 in the form of a powder (min. 92% through 200 mesh (75 μm)) was purchased from Deosen Biochemical Ltd., Ordos, China. Corn starch ULTRA-TEX^® 2131 (pregelatinized, chemically modified starch refined from waxy maize) was supplied by Ingredion Germany GmbH, Hamburg, Germany, and was reported to contain 95.4% carbohydrates, 0.4% protein, and 0.2% salt. All reagents used in the analysis were of analytical grade.

2.2. Sampling

2.2.1. Aquafaba Preparation

Lentil aquafaba was prepared using a traditional two-step process of soaking and cooking in water, either by boiling or under pressure [3]. Briefly, dry lentil seeds were first sorted, and damaged specimens and foreign matter were removed. Then, selected legume seeds (approx. 100 g) were washed and rehydrated by soaking in drinking water in a ratio (to water by weight) of 1:2.5 at a temperature of 15 ± 2 °C for 120 min. After soaking, the water was drained and discarded. Samples of rehydrated lentil seeds (100 g) were washed with drinking water and mixed with 200 mL of drinking water. The samples were then placed in a Thermomix TM6 kitchen appliance (Vorwerk SE & Co. KG, Wuppertal, Germany) and cooked at 99 ± 1 °C for 6–7 min [15]. The cooled LA was drained from the cooked lentil seeds using a stainless steel sieve and stored in a refrigerator at 4 C until further use.

2.2.2. Emulsion Preparation

Ten oil-in-water (O/W) emulsion samples were prepared with different ratios of oil, lentil aquafaba emulsion (LAE), and stabilizers (

Table 1. Formulation in % (w/w) of aquafaba-based emulsion samples.

Sample Code	Oil	Aquafaba	Xanthan	Starch
LAE30	30	70	–	–
LAE30X1/LAE30X2			0.1/0.2	–
LAE30S1/LAE30S2			–	1/2
LAE60	60	40	–	–
LAE60X1/LAE60X2			0.1/0.2	–
LAE60S1/LAE60S2			–	1/2

). The oil phase contained 30 or 60% (w/w) sunflower oil. The protein content was determined by the amount of aquafaba taken according to the formulation.

The influence of the stabilizer concentration on the microstructural and rheological parameters of the emulsion samples was investigated by adding different amounts of xanthan (0.1 or 0.2% w/w) and corn starch (1 or 2% w/w). The samples, regardless of the ingredient ratio, were developed in the same way. First, the required amount of oil (Table 1) was added to the continuous aquafaba phase at 10,000 rpm for 15 min using a high-speed homogenizer IKA T 25 digital ULTRA-TURRAX (KA-Werke GmbH & Co. KG, Staufen, Germany). Then, the corresponding emulsions were thickened through stabilization by adding one of two hydrocolloids in the required concentrations. The final sample was stored in a refrigerator at a temperature of 4 °C. All measurements were performed no earlier than 24 h after the sample was produced.

2.3. Methods

Protein concentration was quantified via spectrometry as in [7]. The UV-absorbance was measured at a wavelength of 562 nm using a UV-1200 spectrometer (Shanghai Mapada Instruments Co., Shanghai, China); bovine serum albumin was used as the reference protein.

The dry matter was measured by moisture analyzer EM 120-HR (Precisa Gravimetrics AG, Dietikon, Switzerland). Measurements were conducted in triplicate.

The apparent viscosity was measured on a rotational viscometer Visco QC 300R (Anton Paar, Graz, Austria) with a concentric cylinder CC12 geometry and a Peltier PTD 175 thermostat device (Anton Paar, Graz, Austria) as in [16]. Steady shear viscosity was determined in the shear rate range of 0.1–100.0 s⁻¹ over a period of 120 s at a temperature of 20 °C. The experimental data on the dependence of apparent viscosity on shear rate were fitted to the power-law, Herschel–Bulkley, and Casson models.

The particle size and droplet size of samples were obtained using a PSA 1190 laser diffraction particle size analyzer (Anton Paar, Graz, Austria) in the range of 0.1–2500 µm as in research [16] without modification. The size analyzer used an optical design for diffraction analysis, which includes multiple lasers for covering the full measurement range, and gives data with and without ultrasound treatment. Particle size distribution was characterized by the De Brouckere mean diameter (volume-weighted mean diameter) D[4,3] and SPAN = (D₉₀ − D₁₀)/D₅₀, where D₁₀, D₅₀, and D₉₀ are the percentile at 10%, 50%, and 90% of the cumulative size distribution, respectively.

The pH of the samples was measured with a 692 pH/Ion meter (Metrohm, Herisau, Switzerland) with the combined glass electrode from Unitrode with Pt1000 (Metrohm, Herisau, Switzerland).

All experiments were performed in triplicate, and results were presented as mean ± standard deviation. One-way ANOVA with Tukey’s multiple comparison post hoc test was performed to assess significant differences between groups at p < 0.05. Statistical data were processed using the SigmaPlot ver. 15 (Grafiti LLC, Palo Alto, CA, USA).

3. Results and Discussion

3.1. Characteristics of Lentil Aquafaba and Emulsions Based on It

It was noted above that lentil aquafaba has a high soluble protein content compared to other legumes. Optimization of the production process resulted in lentil aquafaba with a protein content of 1.06% or approximately 19% on a dry matter basis (

Table 2. Characteristics of lentil aquafaba and emulsions without thickener.

Sample	Protein Content, %	Dry Matter, %	pH	Particle Volume Size Distribution
				D[4,3], µm	SPAN, µm
LA	1.06 ± 0.02	5.5 ± 0.1	6.42 ± 0.01	61.7 ± 0.6	1.71 ± 0.01
LAE30	0.7 *	-	6.39 ± 0.01	13.1 ± 0.3	2.18 ± 0.00
LAE60	0.3 *	-	6.62 ± 0.01	10.5 ± 0.4	1.87 ± 0.00

* Calculated amount based on the protein concentration of aquafaba.

). The protein content data are consistent with those available in the literature when considering the range of 10 to 30% given in the review [5]. Thus, in the study [17], the indicated value of protein content was 1.51% in aquafaba whole green lentils. The authors of [13] estimated the total protein content of aquafaba lentils (Lens culinaris) at 17.1% on a dry matter scale.

The obtained lentil aquafaba was characterized by a dry matter level of 5.5%. This value is higher than that obtained by the authors, i.e., 4.69 [17]. This parameter was the key variable in this study and all prepared emulsions had equal dry matter content. To summarize, these differences in the LA content of dry matter, including protein, may be attributed to various factors, especially lentil variety and processing conditions during aquafaba preparation.

The LA dispersion had a nearly monomodal volume particle size distribution, which was characterized by a mean diameter D[4,3] of 61.7 μm and a SPAN distribution width of 1.71 (Figure 1, Table 2).

The particle size distribution of lentil aquafaba indicates the presence of cell material fragments from the boiled grains. This is entirely predictable, considering the specific characteristics of red lentils, such as the absence of seed coats, short cooking time, and the transformation of whole grains into a puree-like mass. These features result in the accumulation of soluble substances, high-molecular compounds, and solid particles that remain in a suspended state within the aquafaba.

The conducted studies show that depending on the fat content of the emulsion, there are differences in the size distribution of aquafaba particles and droplets of aquafaba-containing emulsions. The emulsion samples are characterized by a pronounced bimodal distribution of droplets. The first peak of the distribution is almost identical for emulsions with a mass fraction of fat of 30% and 60%, and is in the range of 3.5–5.0 µm. However, the second peak was significantly different: for 30% fat emulsions, it was observed in the range of 20–25 µm, while for 60% fat emulsions, it shifted to 10–12 µm. The average droplet size in the 30% fat emulsions was 13.1 ± 0.3 µm, while increasing the fat content to 60% reduced the average droplet size to 10.5 ± 0.4 µm, indicating better homogenization of the resulting systems. The absence of the aquafaba-specific peak in the emulsion distribution curves is attributed to the emulsification process, which promotes the homogenization of both the fat phase and lentil cell particles that transfer into the aqueous phase during hydrothermal treatment.

However, storage of the obtained emulsions over time revealed that the systems are prone to phase separation, indicating that the available protein content is insufficient to stabilize the formed systems. This highlights the relevance of studying the influence of hydrocolloids to obtain emulsions with long-term stability.

3.2. Droplet Size Based on Aquafaba Using Xanthan and Pregelatinized Corn Starch

The obtained lentil aquafaba-based emulsion samples with varying fat content are characterized by low kinetic stability. This necessitates the introduction of additional stabilizers, which enhance the viscosity of the aqueous phase and ensure emulsion stability. Pregelatinized corn starch and xanthan gum were used as emulsion stabilizers. Their implementation makes it possible to produce products with a variety of textures, which is important in terms of the potential technological application of aquafaba in food production. Pregelatinized corn starch has the ability to instantly swell in an aqueous medium without requiring additional thermal treatment. It stabilizes emulsion systems and imparts a thick, creamy texture, preventing phase separation. Xanthan gum, on the other hand, relatively quickly swells and increases viscosity, effectively preventing phase separation. It provides food products containing it with a smooth, gel-like texture, thereby improving their sensory properties and stability.

The droplet size of emulsions is a critical parameter that directly impacts their stability, texture, and sensory perception. The microstructure of food emulsions contains a mixture of different-sized spherical oil droplets, their associates in the form of floccules, and biopolymers not adsorbed onto the droplet surface [18]. The results of studies of the microstructural and rheological properties of emulsions depending on the type of stabilizer are significantly different (

Table 3. Microstructural and rheological properties of emulsion samples.

Sample	Volume Droplet Size Distribution		Power-Law Equation			Herschel–Bulkley Equation
	D[4,3], µm	SPAN, µm	Consistency Index, Pa·sn	Flow Behavior Index	R2	Yield Stress, Pa	Consistency Index, Pa·sn	Flow Behavior Index	R2
LAE30X1	11.3 ± 0.2 d	2.78 ± 0.08 a	1.62± 0.01 b	0.362 ± 0.002 c	0.999	0.22 ± 0.05 b	1.38 ± 0.05 a	0.397 ± 0.007 a	0.999
LAE30X2	14.8 ± 0.4 c	2.49 ± 0.10 b	2.76 ± 0.02 a	0.312 ± 0.003 d	0.998	0.67 ± 0.09 a	2.02 ± 0.09 a	0.380 ± 0.009 b	0.999
LAE30S1	23.7 ± 0.8 b	2.74 ± 0.09 a	0.39 ± 0.01 d	0.565 ± 0.008 a	0.996	<0
LAE30S2	27.3 ± 0.9 a	2.58 ± 0.12 b	1.38 ± 0.02 c	0.466 ± 0.007 b	0.996	<0
LAE60X1	13.2 ± 0.3 c	2.75 ± 0.09 a	5.77 ± 0.18 c	0.306 ± 0.013 c	0.967	3.91 ± 0.33 b	1.31 ± 0.2 b	0.662 ± 0.035 b	0.992
LAE60X2	10.7 ± 0.5 d	2.87 ± 0.08 a	11.7 ± 0.37 a	0.268 ± 0.013 d	0.958	6.79 ± 0.10 a	3.89 ± 0.08 a	0.524 ± 0.004 d	0.999
LAE60S1	19.5 ± 0.9 b	2.81 ± 0.07 a	0.72 ± 0.02 d	0.501 ± 0.013 a	0.988	0.21 ± 0.01 d	0.47 ± 0.01 c	0.644 ± 0.002 bc	0.999
LAE60S2	29.3 ± 0.7 a	2.08 ± 0.10 b	7.75 ± 0.41 b	0.438 ± 0.029 b	0.940	2.93 ± 0.56 c	3.51 ± 0.44 a	0.730 ± 0.036 a	0.995

^a–d Mean values within each column with different superscripts are significantly (p < 0.05) different.

). For aquafaba emulsions with pregelatinized corn starch and xanthan, a bimodal particle distribution is observed, similar to that of aquafaba emulsions without stabilizers.

The position of the first peak is the same for all samples and corresponds to a droplet size of 2.5 µm. The second peak, depending on the sample, occurs in the range of 10–25 µm. This pattern is typical for crude food emulsions obtained by the method described above. The average volume diameter D[4,3] and SPAN are given for the emulsion samples in Table 3. It should be noted that for all emulsions, regardless of the type of hydrocolloid used, an increase in the average droplet size was observed: 1.1–2.0 times for 30% oil emulsions and 1.2–2.8 times for 60% oil emulsions. The polydispersity of all samples was at the same level for both xanthan and starch emulsions.

3.3. Rheological Properties Based on Aquafaba Using Xanthan and Pregelatinized Corn Starch

Apparent viscosity curves in double logarithmic coordinates for samples are presented in Figure 2. The emulsion samples represent structured food systems characterized by a sudden drop in viscosity over a certain range of shear rates. The emulsion samples exhibited shear thinning behavior, which is characteristic of the pseudoplastic flow of non-Newtonian fluids. The emulsions with higher oil content (60%) showed higher apparent viscosity values over the whole range of tested shear rates. This qualitatively corresponds to the functional dependence according to the Einstein equation [18].

The experimental data were fitted within power-law and Herschel–Bulkley models (Table 3). The analysis of the determination coefficient shows the best approximation ability of the Herschel–Bulkley model. For samples with oil content of 30% and starch as stabilizer, the calculation gives a negative result for the yield stress. The value yield stress increases with increasing oil content and with the transition from starch to xanthan. It is well known [19] that the yield strength usually refers to the transitional stress between the behavior of a system as an elastic solid and as a viscous liquid. Fluids with yield strength behave as a solid until a minimum stress is overcome to initiate material flow. This solid behavior is a manifestation of the viscoelastic behavior of structured systems. The shear thinning strength of the structure is governed by both the interactions of the dispersed phase droplets and the influence of the stabilizing effect from the introduced stabilizer. The data in Table 3 confirm these statements by presenting a higher viscoelasticity of emulsions with a higher oil content due to a denser oil network. It should also be stated that xanthan with a concentration of 0.2% is the most effective stabilizer. However, the structure formed in this case can lead to more rigid textures, which are not always suitable for the criteria of the sensory characteristics of the final product. In this case, other combinations can be used.

4. Conclusions

This study successfully optimized the production of lentil aquafaba, achieving a protein content of 1.06% and a dry matter concentration of 5.5%. This type of aquafaba demonstrated functional properties suitable for creating emulsions, showing bimodal droplet size distributions and pseudoplastic flow behavior characteristic of non-Newtonian fluids. Emulsions stabilized with xanthan gum exhibited smaller droplet sizes but the same polydispersity compared to those with starch, highlighting its superior stabilizing properties. Additionally, increasing the oil content in the emulsions (from 30% to 60% w/w) led to higher apparent viscosity values, consistent with theoretical predictions for concentrated emulsions. Rheological analysis revealed that the Herschel–Bulkley model provided the best fit for describing the flow behavior of these systems, validating its applicability to non-Newtonian food emulsions. These findings underline the potential of lentil aquafaba as a sustainable emulsifier and offer valuable insights for the formulation of low-fat food products with enhanced stability and functional properties.